This invention relates to optical recording mediums and, more particularly, to an optical recording medium having a recording layer containing an organic coloring matter.
Ordinarily, optical recording mediums have superior characteristics; they have a large storage capacity and are capable of effecting writing or reading in a non-contact manner. Optical recording mediums have therefore been developed extensively.
A write once type optical disk is known as an example of optical recording mediums. This type of optical disk is used in such a manner that a laser beam is condensed to a very small area on a recording layer and is converted into thermal energy to change the state of the recording layer (forming pits) thereby to write data, and that the corresponding data is reproduced in accordance with changes in the quantity of light reflected on recording portions and non-recording portions.
Preferably, the recording layer of such a medium is formed of a material capable of achieving a large change in reflectivity. Low melting point metallic materials containing tellurium have previously been used to form the recording layer.
Recently, however, optical recording mediums having a recording layer formed of an organic material mainly constituted by a coloring matter have been proposed for use in place of tellurium materials because tellurium materials are toxic, and because it is necessary to improve the sensitivity of medium and to reduce the manufacture cost. (Examples of such mediums are disclosed in Japanese Patent Laid-Open Publication Nos. 58-112790, 58-114989, 58-125246 and 60-71295.)
In general, an optical recording medium having a recording layer formed of an organic material mainly constituted by a coloring matter has a substrate, an ultraviolet-curing resin layer (photo-polymer layer) formed on the substrate to form grooves and addresses, and a recording layer formed on the ultraviolet-curing resin layer by spin coating.
The substrate having the photo-polymer layer is formed as described below. The ultraviolet-curing resin is dropped on, for example, a Ni stamper, the substrate is placed thereon, and the ultraviolet-curing resin is spread so as to have a sufficiently small thickness while preventing formation of bubbles between the stamper and the substrate. Thereafter, the ultraviolet-curing resin is irradiated with ultraviolet rays through the substrate to be polymerized and cured. Therefore the process for forming this substrate is complicated and troublesome and it is difficult to improve the productivity. To avoid these problems, an injection-molded substrate may be used which is a plastic substrate integrally formed with grooves and so on by injection molding without using any ultraviolet-curing resin.
This injection-molded substrate, however, cannot be used in practice because it has a recording layer directly formed by coating on its surface on the groove side) and is therefore damaged generally by a solvent contained in the coating liquid.